(1) Field of the Invention
The invention relates to field emission devices in general and, more particularly, to their design and manufacture.
(2) Description of the Prior Art
Cold cathode electron emission devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small conical emitters, each of which is connected to a source of negative voltage via a cathode conductor line or column. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines and is orthogonally disposed relative to them, intersecting with them at the locations of the conical emitters or microtips, and connected to a source of positive voltage.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike an cathodoluminescent panel that is located a short distance from the gate lines. In general, a significant number of microtips serve together as a single pixel for the total display. Note that, even though the local electric field in the immediate vicinity of a microtip is in excess of 10 million volts/cm., the externally applied voltage is only of the order of 100 volts.
In FIG. 1 we show, in schematic cross-section, the basic elements of a typical cold cathode display. Metallic lines 2 are formed on the surface of an insulating substrate 1. Said lines are referred to as cathode columns. At regular intervals along the cathode columns, microtips 5 are formed. These are typically cones of height about one micron and base diameter about one micron and comprise molybdenum or silicon, though other materials may also be used. In many embodiments of the prior art, local ballast resistors (not shown here) may be in place between the cones and the cathode columns.
Metallic lines 4 are formed at right angles to the cathode columns, intersecting them at the locations of the microtips. A layer of insulation 3 supports lines 4, which are generally known as gate lines, placing them at the top level of the microtips, that is at the level of the apexes 9 of the cones 5. Openings 11 in the gate lines 4, directly over the microtips, allow streams of electrons to emerge from the tips when sufficient voltage is applied between the gate lines and the cathode columns. Because of the local high fields right at the surface of the microtips, relatively modest voltages, of the order of 100 volts are sufficient.
After emerging through the openings 11 in the gate lines, electrons are further accelerated so that they strike a fluorescent screen (not shown) where they emit visible light. Said fluorescent screen is separated from the cold cathode assembly by spacers (not shown) and the space between these two assemblies is evacuated to provide and maintain a vacuum of the order of 10.sup.-7 torr.
Continuing our reference to FIG. 1, note should be made of several important design parameters. The smaller the distance R, that is the distance between microtip apex 9 and gate electrode 4, the greater the electric field at 9 and hence the greater the electron emission current for a given applied voltage. The methods normally used for manufacturing the microtips (see below) lead to a geometry wherein the diameter D of the well or cavity 11, inside which the microtips are formed, is approximately equal to the depth H of the cavity. Thus, to minimize R it is normally necessary to similarly minimize H. However, the capacitance between a pair of intersecting cathode and gate lines (shown schematically as C) is inversely proportional to H so a reduction in R is normally associated with an increase in C. Such an increase in capacitance is undesirable because it causes an increase in power consumption and in the time constant of the emitter.
The conventional method for forming microtips has been described by Spindt in U.S. Pat. No. 5,064,396 Nov. 1991 and is schematically illustrated in FIG. 2. Cathode lines 2 are formed on substrate 1. Gate lines 4 are formed so as to run at right angles to lines 2, being separated from them by insulating layer 3. Well or cavity 11 is formed in layer 3 by first forming an opening in gate layer 4 and then using 4 as a mask for the etching of 3. Etching is continued past the minimum time needed to reach layer 2 so a certain amount of undercutting of layer 4 at the top of 11 occurs.
The entire assemblage is now placed in a vacuum chamber and rotated about a centrally located vertical axis, such as 6. While rotation occurs, an evaporant beam 7 of a suitable release material (also called a sacrificial material), such as silicon oxide, is directed at the surface of 4 at close to grazing incidence. This begins a gradual buildup of release material 8 at the entrance to cavity 11. Then, with evaporant beam 7 still active, a vertically directed beam 12 of material suitable for a microtip emitter (such as molybdenum or silicon) is started. Material from beam 12 then builds up inside cavity 11 in the shape of cone 5 because the entrance to 11 is gradually closing.
Once the closure of 11 is complete the evaporation processes are terminated and the entire structure is allowed to soak in an etchant that selectively attacks release material 8 and nothing else. As material 8 disintegrates, material from 12 that had been deposited so as to be in contact with 8 get lifted off and eventually completely removed so that the finished device has the appearance seen in FIG. 1.
Returning now to the design problems mentioned above, a solution has been provided by Spindt (U.S. Pat. No. 5,064,396). In his process, layer 3 (in FIG. 1) is initially made twice as thick, other dimensions remaining unchanged. Cone 5 is then formed as described above resulting in a microtip whose height is half that of the cavity. After liftoff, the entire cone formation process is repeated once again, resulting in a tall slim cone whose apex is once more at the level of gate layer 4. While this method cuts the capacitance in half without reducing emission current, it significantly increases the cost of manufacturing the device. There is also some increase in the series resistance associated with the use of a longer slimmer cone.
Holmberg (U.S. Pat. No. 5,075,591 Dec 1991) avoids the use of two cone formation steps. Instead, two layers of insulation are used Co form the equivalent of layer 3 (FIG. 1), the topmost of these being removed only in the immediate vicinity of cavity 11. Layer 4 is formed so as to lie mainly on both insulation layers but does extend all the way to openings 11. Thus most of capacitor C derives its magnitude from the thickness of the double insulation layer. This approach, however, also increases the manufacturing costs. Additionally, the geometry of the structure limits the extent to which evaporant beams may be effectively used at grazing incidence as well as introducing the possibility of discontinuties in layer 4 at the step down to the cavity.
Kane (U.S. Pat. No. 5,320,570 Jun 1994) provides a solution wherein only a single cone formation step is used but, rather than having the cones lie directly on layer 2 (FIG. 1), insulating layer 3 is made thicker and the depth of cavity 11 is extended. A column, or pedestal, is then provided for the cone to rest on so that the cone remains electrically connected to cathode line 2 while its apex is raised so as to be level with the opening of cavity 11. In Kane's structure, both the pedestals and the cathode lines are formed from the same initial layer of metal. This makes it difficult to impossible to, for example, provide individual ballast resistors for the microtips. Additionally, since the pedestals are formed prior to the formation of the openings in gate lines 4, a very precise alignment step is needed to ensure that each cavity 11 is exactly lined up with its intended pedestal.
Thus, the structure that is the end product of the process of the present invention is similar to that of Kane in that pedestals are also used to support the cones inside a deeper cavity. However, the process of the present invention is simpler and more flexible than Kane's process.